CIRCUITS!!

TEST MAY 9!!!!

ELECTRIC CURRENT (I)

• The movement of charge.

• SI unit is the Ampere (A, Amps)

• Two types: direct and alternating

• Direct – batteries

• Alternating – wall outlets, light fixtures

•We use “conventional current”

• The direction of movement of a proton

• EVEN THOUGH we know protons don’t move!!!!

𝐼 =𝑞

𝑡

EXAMPLE: A STEADY CURRENT OF 2.5A EXISTS IN A WIRE FOR 4.0 MIN. A. HOW MUCH CHARGE PASSES A POINT IN THE CIRCUIT?B. HOW MANY ELECTRONS IS THIS?

TASK: USE THE WIRES, LIGHT, BATTERY AND SWITCH. ATTACH ALL OF THESE PARTS TO MAKE THE BULB LIGHT.

• 1. Draw a picture of your setup

• 2. Open the switch. What happens?

• 3. Switch the battery (turn it around). What happens?

• 4. Can you tell which way current is flowing?

CIRCUIT – A CONTINUOUS CONDUCTING PATH CONNECTED BETWEEN THE TERMINALS OF A BATTERY

• Circuit – comes from the word “circle”

• Why does this make sense?

• Closed circuit – continuous conducting path , current flows

• Open circuit - a break in the path; no current flows

• Instead of a picture – we draw circuit

diagrams with symbols

• Cause THIS AINT ART!!!!

• Always in a block or triangle

FLOW OF CURRENT IN A CIRCUIT:

• For conventional current:

• Negative terminal has 0 voltage

• As charge flows to + terminal, voltage is gained

• Wires are considered perfect conductors (THEY AREN’T

THOUGH!!!) so they don’t take away voltage.

• Resistors and bulbs take voltage

• When the charge reaches the negative terminal again,

there is NO voltage

OHM’S LAW – IN METAL CONDUCTORS RESISTANCE (R) IS A CONSTANT INDEPENDENT OF POTENTIAL DIFFERENCE (V)

• Resistance (R) – friction of a circuit

• Slows down current

• Measured in Ohms (Ω)

• Decreases current

• Not all resistors are “Ohmic”

• We engineer resistors to behave differently at different voltages (Semiconductors)

• “Ohmic” resistors have a linear relationship!!

𝑉 = 𝐼𝑅

EXAMPLE: A small flashlight bulb draws 300 mA from its 1.5 V battery.

• A. What is the resistance of the bulb?

• B. If the battery becomes weak and the voltage drops to 1.2 V, what would

happen to the current?

ELECTRIC POWER (P) -

• Measured in Watts (W)

• How else could we write the equation, using Ohm’s Law?

• Example: Calculate the resistance of a 40.0 W headlight designed for 12 V.

𝑃 = 𝐼𝑉

HOW DOES THE ELECTRIC COMPANY CHARGE?

• An electric heater draws a steady 15.0 A on a 120 V line. How much power

does it require?

• How much does it cost per month if it operates for 3.0 hours per day and the

electric company charges 9.2 cents per kWh?

RESISTORS IN PARALLEL AND SERIES

• Parallel

• Current splits at junctions

• Same voltage across each branch

• Series

• Connected end to end

• Same Current throughout

1

𝑅𝑒𝑞=

1

𝑅1+

1

𝑅2+⋯

𝐼𝑇 = 𝐼1 + 𝐼2 +⋯

𝑅𝑒𝑞 = 𝑅1 + 𝑅2+…

𝑉𝑇 = 𝑉1 + 𝑉2 +⋯

EXAMPLE 1:

1. Series or parallel?

2. Calculate equivalent

resistance.

3. Calculate potential

difference and

current across each

resistor.

EXAMPLE 2

1. Series or parallel?

2. Calculate equivalent

resistance.

3. Calculate potential

difference and

current across each

resistor.

EXAMPLE 3:

1. Series or parallel?

2. Calculate equivalent

resistance.

3. Calculate potential

difference and

current across each

resistor.

EXAMPLE 4:

1. Series or parallel?

2. Calculate equivalent

resistance.

3. Calculate potential

difference and

current across each

resistor.

COMPLEX CIRCUITS – CONTAIN SERIES AND PARALLEL ELEMENTS• Combine parallel elements first – replace with equivalent resistor

• Redraw your diagram each time you combine resistors.

• Work backward to find I and V for each element.